为什么“A <0> = 0”中的template-id由于大于或等于运算符“> =”而无法在没有空格的情况下进行编译？

Question

template <int>
using A = int;

void f(A<0>=0);   // Attempting to declare a function f taking int,
                  // with the default argument 0

// Works as expected:
// void f(A<0> = 0);

This neither compiles on GCC 4.9.2 nor Clang 3.5 - let alone ICC or VC++. Apparently the >= bit is parsed as a greater-or-equal-than operator. However, this seems to be incorrect regarding [temp.names]/3:

After name lookup (3.4) finds that a name is a template-name or that an operator-function-id or a literal- operator-id refers to a set of overloaded functions any member of which is a function template, if this is followed by a < , the < is always taken as the delimiter of a template-argument-list and never as the less-than operator. When parsing a template-argument-list , the first non-nested > ¹³⁸ is taken as the ending delimiter rather than a greater-than operator. [..] [ Example:

 template<int i> class X { /* ...*/ }; X< 1>2 > x1; // syntax error X<(1>2)> x2; // OK 

— end example ]

^{138) A > that encloses the type-id of a dynamic_cast , static_cast , reinterpret_cast or const_cast , or which encloses the template-argument s of a subsequent template-id , is considered nested for the purpose of this description.}

Am I missing something or is this a compiler bug?

Answer 1

This an effect of the maximal munch principle , which has the lexical analyzer take as many characters as possible to form a valid token. This is covered in draft C++ standard section 2.5 [lex.pptoken] which says:

Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail.

Any cases such as the one you cite above need a specific exception carved out such as this case for <:: , we can see an example in the following code:

template<typename T> class SomeClass;
class Class;

SomeClass<::Class>* cls;

which is covered in this question , the exception is listed in the bullet just above the maximal munch rule:

Otherwise, if the next three characters are <:: and the subsequent character is neither : nor >, the < is treated as a preprocessor token by itself and not as the first character of the alternative token <:.

and of course the non-nested > which you cite in your question.

Note we can see that >= is a preprocessor token from section 2.13 [lex.operators] which says:

The lexical representation of C++ programs includes a number of preprocessing tokens which are used in the syntax of the preprocessor or are converted into tokens for operators and punctuators:

and includes >= in the list.

We can see from the proposal that fixed the >> case: N1757: Right Angle Brackets which says ( emphasis mine ):

Ever since the introduction of angle brackets, C++ programmers have been surprised by the fact that two consecutive right angle brackets must be separated by whitespace:

 #include <vector> typedef std::vector<std::vector<int> > Table; // OK typedef std::vector<std::vector<bool>> Flags; // Error 

The problem is an immediate consequence of the the “maximum munch” principle and the fact that >> is a valid token (right shift) in C++.

This issue is a minor, but persisting, annoying, and somewhat embarrassing problem. If the cost is reasonable, it seems therefore worthwhile to eliminate the surprise.

The purpose of this document is to explain ways to allow >> to be treated as two closing angle brackets, as well as to discuss the resulting issues. A specific option is proposed along with wording that would implement the proposal in the current working paper.

Also points out the >= case:

It is also worth noting that the problem can also occur with the >>= and >= tokens . For example

 void func(List<B>= default_val1); void func(List<List<B>>= default_val2); 

Both of these forms are currently ill-formed. It may be desirable to also address this issue, but this paper does not propose to do so.

Note, this change broke backward compatibility with C++03 .